A trend toward the miniaturization such as of portable electronic devices, etc., has entailed most developments of lithium ion cells that are high in energy density and are capable of being miniaturized.
For the outer sheathing material of lithium ion cells, metal cans have hitherto been in frequent use. In recent years, multilayer films, which are lighter in weight and excellent in heat dissipation, have been employed.
For an electrolytic solution of lithium ion cell, there is used an aprotic solvent such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate or the like, and an electrolyte such as a lithium salt including LiPF6, LiBF4 or the like. These lithium salts react with water to form hydrofluoric acid, thereby causing the electrolytic solution to be degraded and cell members to be corroded. To cope with this, the outer sheathing material made of a multilayer film is usually provided with an aluminum foil layer in the inside thereof. For instance, there is known an outer sheathing material of the type, which has a base material layer/adhesive layer/aluminum foil layer/corrosion inhibition treatment layer/adhesive resin layer/sealant layer laminated successively.
In order to supply electric power from a lithium ion cell making use of the outer sheathing material formed of such a multilayer film, metal terminals called tabs, which are connected to positive and negative electrodes, respectively, are necessary. The tab is made of a metal terminal (lead) and a metal terminal covering resin film (tab sealant), and it is usual that aluminum is used for a lead (connected to a positive electrode) at a positive electrode side and nickel or copper is used for a lead (connected to a negative electrode) at a negative electrode side. The tab sealant is a member interposed between the lead and the outer sheathing material and should have the following properties.
For one of the properties required for the tab sealant, mention is made that it has adhesion to both the lead and the outer sheathing material. The adhesion between the lead and the outer sheathing material can be secured by use of a thermal fusible resin. The adhesion between the tab sealant and the lead can be improved by acid modification of a polyolefin resin used for the tab sealant layer.
Further, in the case where the tab sealant is fusion-bonded to the lead, a lead end portion has to be filled with the tab sealant without space therebetween. If the filling of the lead end portion with the tab sealant is insufficient, a space is formed between the tab sealant and the lead, thereby causing a content to be leaked out or the tab sealant and the lead to be separated from each other.
For the second property required for the tab sealant, mention is made that insulation between the lead and other member is secured. Upon thermal fusion bonding between the tab sealant and the lead, there is the possibility that although depending on the pressure and temperature conditions, the thickness of the tab sealant fusion-bonded to the lead becomes so small as not to secure the insulation. Especially, the thickness of the tab sealant fusion-bonded at the shoulder (or angled potion) of the lead is apt to be smallest. To avoid this, it is necessary to make the resin harder to flow by lowering its melt flow rate or to make use of a high melting resin so as to render it difficult to melt part of the tab sealant.
For example, in Patent Literature 1, acid-modified polypropylene is used as a tab sealant to improve adhesion between the tab sealant and a lead, and a tab sealant is formed as a three-layer structure wherein an intermediate layer of the tab layer is made higher in melting point than the other layers so as to keep the film thickness, thereby securing insulation.